Rebar Grid Calculator (Imperial Bar Sizes)
US bar size and spacing to counts, total length and pounds.
Also known as rebar spacing · bar spacing grid · mesh spacing
With the default inputs: Length 20 ft · Width 10 ft · Bar size #4 etc.Total rebar length 946 ft
Use your own dimensions ↓Inputs
Result
| Bars running the length | 11bars |
| Bars running the width | 21bars |
| Net length per layer | 430ft |
| Layers | 2levels |
| Net total length | 860ft |
| With laps | 946ft |
| Unit weight | 0.668lb/ft |
| Total weight | 632lb |
| Short tons | 0.316ton |
How this is worked out
Bar count is floor(span × 12 ÷ spacing) + 1 - the plus one is the starter bar at the edge and it is the item most often missed. A single layer only controls temperature and shrinkage; structural mats are two-way, two layers, so picking the wrong layer count halves or doubles the order. Lap length in ACI terms depends on bar size (roughly 40 bar diameters in tension before modification factors), and this tool applies your lap as a percentage of total length rather than segment by segment - good for ordering, not a substitute for the placing drawing. Weights are ASTM nominal; do not mix them with metric bar weights.
Next up: Rebar Weight Calculator · Rebar Anchorage Length Calculator
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| count = floor(span(ft) × 12 ÷ spacing(in)) + 1 | the +1 is the edge bar |
| net length = Σ(count × span) for both directions | |
| total = net × layers × (1 + lap%) | |
| weight = total × unit weight | #3 0.376 / #4 0.668 / #5 1.043 / #6 1.502 lb/ft |
How to work it out
- Fix the bar size and spacing: #4 @ 12 in o.c. reads as number 4 bars at 12 in on centre.
- Count = floor(span in the run direction × 12 ÷ spacing) + 1, worked out for both directions.
- Multiply each direction's count by its span and add, giving the net length per layer.
- Multiply by layers - a single layer only controls temperature and shrinkage, structural mats are two-way two-layer.
- Add laps (10-15% for ordering) and multiply by that bar size's nominal unit weight for pounds.
Where it goes wrong
- Skipping the plus one - the edge bar goes missing, which is 25% short on a narrow element.
- Dividing a span in feet by spacing in inches - 20 ft over 12 gives 1.67 bars, which is absurd but common.
- Treating a single layer as structural - mats need top and bottom, so the quantity doubles.
- Multiplying an imperial length by a metric bar weight - the units are mixed and the number cannot be checked.
Material and cost
Rebar is sold in 20 ft and 40 ft stock lengths, or cut and bent to order. Prices track steel and move a lot: #4 commonly runs USD 0.9-1.4 per foot and #5 about USD 1.4-2.0 per foot in 2026, varying widely by region, with bulk weight pricing around USD 900-1200 per ton. Add tie wire, chairs at 0.3-0.6 each, and tying labour, which on residential work often matches the material cost.
On site
- Plan orders around stock lengths - a 20 ft span is cheapest in 20 ft bars, and cutting 40 ft stock wastes the offcut.
- Set chairs about every 4 ft and keep boots off the mat; stepping it down 2 in destroys the cover.
- Stagger laps - where more than half the bars splice at one section, lap length has to increase.
Basis
- ACI 318 Building Code Requirements for Structural Concrete (reinforcement, spacing and laps)
- ASTM A615 Standard Specification for Deformed and Plain Carbon-Steel Bars (nominal size and unit weight)
- CRSI Manual of Standard Practice (fabrication and placing practice)
FAQ
What size is #4 rebar?
Why is there a plus one on the bar count?
What lap percentage should I use?
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Rebar Grid Calculator (Imperial Bar Sizes). 2026-10-25. https://jianzhujisuan.com/en/rebar-grid